Kick-down switching speed optimization for an automatic transmission of a motor vehicle

ABSTRACT

In the method for kick-down switching speed optimization in a motor vehicle with an automatic transmission, the kick-down upshift point is determined as a function of the load conditions and the road inclination in each case.

[0001] The invention concerns a method for kick-down switching speedoptimization in a motor vehicle with an automatic transmission,according to the pre-characterizing portion of Claim 1.

[0002] For changing gear in an automatic transmission a certain delaytime is allowed for before the switching is completed. During this, theengine speed increases until the load is taken up by the coupling beingengaged from the coupling being disengaged. The reasons for the delaytime are clutch filling times and ramp times until the switchingpressure required for the change has been built up.

[0003] In vehicles with powerful engines the engine speed differencethat occurs, having regard to the clutch filling times including theload take-up, can be as much as about 1200 r/min.

[0004] The maximum attainable engine speed in kick-down upshift gearchanges can therefore assume various values which are affected by theload condition of the vehicle and the road inclination.

[0005] The problem arises that in an unladen vehicle moving downhill,kick-down (KD) upshifts take place in the range of the engine cut-offspeed, i.e. the maximum permissible engine speed. The reason for this isthe negative driving resistance, by which the vehicle is additionallyaccelerated.

[0006] In such a case the kick-down upshift must take place and adjustedearlier, i.e. at lower output speed. In the loaded condition and drivinguphill, in contrast, the KD switching speed is lower as a result.

[0007] Accordingly, the optimum condition, namely equal KD switchingspeed for any kick-down upshift, cannot be achieved.

[0008] In the prior art kick-down shifts are triggered whenpredetermined output speeds are exceeded. These output speed thresholdscan be stored in switching programs or be defined as discreteparameters.

[0009] The purpose of the present invention, starting from the aforesaidprior art, is therefore to provide a method for kick-down switchingspeed optimization in a motor vehicle with an automatic transmission,such that in kick-down upshifts the engine is prevented from exceedingthe maximum permissible speed. In addition, premature upshifting is tobe prevented.

[0010] According to the invention this objective is achieved by thecharacteristics of Claim 1. Other features of the invention emerge fromthe subordinate claims.

[0011] Accordingly, it is proposed to determine the kick-down upshiftpoint adaptively, i.e. as a function of the respective load conditionsand road inclinations, so that switching takes place at a desiredmaximum engine speed.

[0012] According to the invention, when a kick-down condition isdetected by the transmission control system, a speed offset nd_abkd isadded to the current upshift point. This speed offset is of appropriatesign and is stored in the transmission control system in the form of acharacteristic line, a separate characteristic line being stored foreach upshift.

[0013] According to a variant of this invention, when a kick-downcondition has been recognized the target gear of the next upshift andthe transmission output speed gradient are determined.

[0014] Thereafter, the speed offset nd_abkd is calculated. For thispurpose the delay times for the individual gear shifts are stored forapplication.

[0015] In an advantageous variant, the value of the speed offset isdetermined in the form of a characteristic line in accordance with bothof the above methods and then recalculated as a function of the driver'sactivity (for example, by means of a valuation counter), so that n_abkdwill be higher in a KD-upshift by a sporty driver than in the case of amore sparing driver.

[0016] According to this variant, the characteristic line is multipliedby a factor that characterizes the driver's activity as a function ofthe gear change and output speed gradient. In this case thecharacteristic line always gives positive values. Alternatively,different characteristic lines are established for variouscharacteristic driver behaviors (again as a function of the gar changeand output speed gradient). By averaging between the driver types,intermediate types of drivers can be allowed for.

[0017] Below, the invention is explained in greater detail withreference to the figures, which show:

[0018]FIG. 1 is a time-engine speed (n_mot-t-) diagram, whichillustrates the problem upon which the invention is based, and

[0019]FIG. 2 is a representation of the speed offset as a function ofthe output speed gradient according to the invention

[0020] As illustrated in FIG. 1, when a gear shift is triggered byreaching a certain predetermined switching speed, the delay time Δt forthe gear change can be longer than the time remaining before the maximumpermissible engine speed is reached. This is particularly the case whendriving downhill, when because of the negative road inclination thevehicle is accelerated additionally. This causes the maximum enginespeed to be exceeded and is represented by drive A in FIG. 1. Incontrast, an optimum kick-down upshift is illustrated by curve B. In thelatter case the shift takes place at the maximum engine speed butwithout exceeding it.

[0021] On the example of a gear upshift, FIG. 2 shows the engine speedoffset according to the invention as a function of the output speedgradient ng_ab.

[0022] When a kick-down condition is recognized by the transmissioncontrol system, a speed offset nd_abkd of appropriate sign is added tothe current upshift point.

[0023] On the example of the upswitch shown in FIG. 2, the value of thespeed offset nd_abkd is equal to zero at a certain output speed gradientng_ab (here for example 450 r/min per second). At a higher ng_ab value(for example, when driving downhill) the speed offset nd_abkd accordingto the invention will assume a negative value, i.e. the gear change willbe triggered at a lower output speed. In the case when the output speedgradient is smaller than a predetermined value, the speed offset nd_abkdis positive, i.e. the gear change takes place at a higher output speedvalue.

[0024] The variation of the speed offset of appropriate sign is storedin the transmission control system in the form of a characteristic line,and for each upshift X a separate characteristic line KL_ND_ABKDX isstored:

nd _(—) abkdX=KL _(—) ND _(—) ABKDX [ng _(—) ab]

[0025] By virtue of this procedure the kick-down upshift point is madeadaptive, i.e. it is determined as a function of the respective loadconditions and road inclinations, so that the gear change takes place ata desired maximum engine speed.

[0026] Alternatively, in a variant of the present invention, instead ofa speed offset an absolute kick-down switching characteristic line canbe used.

[0027] According to a further variant of this invention, when akick-down condition has been recognized the target gear of the nextupshift and the transmission output speed gradient are determined.

[0028] Then, the speed offset nd_abkd is calculated. For this purpose,the delay times for the individual gear changes are stored forapplication. This procedure has the advantage that temperature-dependentdelay times are taken into account when calculating the speed offsetnd_abkd, as indicated by the following equation which shows an exampleof a possible calculation process: nd_abkd=ng_ab*KW_TD_KDX [CGT] withCGT as the transmission temperature and KW_TD_KD as the characteristicline of delay times for individual gear changes.

[0029] For the adaptive kick-down upshift speed for a particular upshiftX, this gives:

n _(—) abkdX=KW _(—) ND _(—) ABKDX−nd _(—) abkd.

1. Method for kick-down switching speed optimization in a motor vehiclewith an automatic transmission, characterized in that the kick-downupswitch point is determined as a function of the load conditions androad inclination in each case.
 2. Method according to claim 1,characterized in that when a kick-down condition is recognized by thetransmission control system, a speed offset of appropriate sign(nd_abkd) is added to the current upshift point as a function of theoutput speed gradient (ng_ab).
 3. Method according to claim 2,characterized in that the variation of the speed offset of appropriatesign (nd_abkd) is stored in the transmission control system in the formof a characteristic line a separate characteristic line being stored foreach upshift.
 4. Method according to claim 1, characterized in that anabsolute kick-down switching characteristic line is used for thedetermination of the kick-down upshift point.
 5. Method according toclaim 1, characterized in that when a kick-down condition is recognized,the target gear for the next upswitch and the transmission output speedgradient (ng_ab) are determined and the speed offset (nd_abkd) is thencalculated, the delay times for individual gear changes being stored forapplication and temperature-dependent delay times being taken intoaccount.
 6. Method according to any of claims 2 to 6, characterized inthat the value of the speed offset (nd_abkd) is calculated, and is thenrecalculated as a function of the existing driver behavior, in suchmanner that the upshift speed (n_abkd) is adapted to the driver's way ofdriving.
 7. Method according to claim 6, characterized in that the valueof the speed offset (nd_abkd) is re-calculated as a function of driveractivity by multiplying the characteristic line (nd_abkd) by a factorthat depends on driver behavior.
 8. Method according to claim 6,characterized in that the value of the speed offset (nd_abkd) isre-calculated as a function of driver activity by establishingcharacteristic lines for each characteristic type of driver,intermediate values being determined by averaging between the drivertypes.